1. Field of the Invention
This invention relates to the inhibition of corrosion in molten alkali metals of nickel-containing alloys such as nickel-based alloys, austenitic stainless steels, long-range ordered alloys, etc.
Though nickel-based alloys and stainless steels are resistant to many types of corrosive attack, there are environments in which corrosion can be a serious problem. For example, molten alkali metals such as sodium and lithium are known to attack nickel-containing alloys to a measurable degree. The corrosive interaction of these alloys and metals are of particular interest in fusion reactor technology wherein nickel-based alloys and austenitic stainless steels have been proposed for major structural elements, cooling system components, and first wall assemblies for isolating the fusion plasma from the surrounding neutron-absorbing blanket. Molten lithium, or lithium-containing compounds, have been designated as the most suitable blanket material because of the properties of lithium as a coolant, neutron absorber, and tritium fuel breeder. Molten alkali metals have also been suggested as external reactor media to remove heat for conversion to a usable energy form by means of a conventional thermodynamic cycle. Since these applications involve contact of nickel-containing alloys and molten alkali metals under conditions conducive to corrosive attack, there has been a reluctance to select nickel-containing alloys because other structural candidates have more favorable corrosion resistance. A detailed analysis of such considerations can be found in Fraas, A. P., "Conceptual Design of the Blanket and Shield Region and Related Systems for a Full Scale Toroidal Fusion Reactor," ORNL-TM 3096, Oak Ridge National Laboratory, Oak Ridge, Tenn. (May 1973), which is incorporated herein by reference. Copies of this publication are available from the National Technical Information Service, U.S. Department of Commerce, Springfield, Va.
Another environment in which nickel-containing alloys could be attacked by molten alkali metals is in a thermodynamic cycle for steam generation from a nuclear reactor. Such an apparatus is described in commonly assigned U.S. Pat. No. 4,072,183 issued to Arthur P. Fraas for "Heat Exchanger with Intermediate Evaporating and Condensing Fluid."
Corrosion of nickel-containing alloys is a problem in other environments such as those associated with molten salts used in coal liquefaction processes, batteries, and in nuclear reactor systems employing a circulating fused fluoride salt as fuel. Corrosion-resistant materials are necessary in these applications to contain the aggressive corrosion behavior of the alkali metals without adversely affecting the nuclear properties of the system.
2. Description of the Prior Art
A widely used corrosion prevention method is the application of corrosion-resistant coatings to metals. In the prior art protective aluminum coatings have been applied by physically covering a surface with an aluminum-containing powder and heating in a reducing environment to about 1000.degree. C. to cause the deposition of aluminum. Such processes which involve the packing of solid material (known as Pack processes) are generally not applicable to complex geometries. Typically, individual parts of complex apparatus are treated prior to assembly and critical joint areas such as weldments are left unprotected in the assembled apparatus.
A fusion reactor application of a similar process wherein aluminum or beryllium oxides are plasma sprayed onto a structural material is described in commonly assigned U.S. Pat. No. 4,004,890 entitled "Method and Means of Reducing Erosion of Components of Plasma Devices Exposed to Helium and Hydrogen Isotope Radiation," issued Jan. 25, 1977.
Another method for applying protective coatings to metals is the deposition of metallic coatings from a molten salt. One such process is described in commonly assigned U.S. Pat. No. 3,783,014 entitled "Electroless Coating of Molybdenum on Stainless Steels," issued Jan. 1, 1974. The deposition of coatings from liquid metals was described by O. M. Zbozhnaya et al. in "Preparation of Diffusion Coatings by Isothermal Mass Transfer in Molten Metals," Fiz.-Khim. Mekh. Mater. 1973 9(4), pp. 43-48, Chemical Abstracts, 51010N. These workers report the isothermal mass transfer of Be, B, Al, V, Cr, Ga, Ge, Ni, Nb, Mo, W, and Pt from molten Ca, Li, Na, Bi, Pb, and Cd alloys at 800.degree.-1000.degree. C. onto Armco iron. The integrity of the coatings was not described.
Yet another deposition process for producing protective coatings from alkali metals is described in U.S. Pat. No. 3,481,770 issued Dec. 2, 1969, in the name of Charles H. Lemke entitled "Process for Preparing Alloy Diffusion Coatings" wherein metal substrates are immersed in baths to obtain coatings which are not fully characterized.
In the foregoing prior art processes, the protective coating, once formed, is sacrificed during corrosive service without further attempts to heal or maintain the initial coating at a desired thickness. The maintenance of such a protective coating would tend to ameliorate. reduce, or eliminate the corrosive effects of alkali metals without adversely affecting their characteristic properties.